The present invention relates to an image forming apparatus capable of forming an erasable image and, more particularly, to an image forming apparatus for forming an image on a sheet by a decolorizable toner.
There has recently been reported a decolorizable toner which becomes transparent when illuminated, heated or otherwise treated. For example, a toner in which a functional infrared absorptive coloring matter IR820B and ammonium salt of organic boron, e.g., tetrabutyl ammoniumbutyl triphenylborate are coexistingly dispersed is taught in "Functional Coloring Matter Sectional Meeting, Data No. 17", Functional Coloring Matter Sectional Meeting of the Kinki Institute of Chemical Engineers, May 29, 1991. A decolorizable toner reactive to light appears blue before decolorization and exhibits the maximum absorption for a wavelength of 825 nm. When illuminated by infrared rays having a wavelength range close to 825 nm or light containing such infrared components (e.g. light from a halogen lamp), the decolorizable toner becomes transparent. Therefore, by decolorizing a decolorizable toner image existing on a sheet, it is possible to use the sheet repetitively. This effectively saves limited natural resources. Let the toner made transparent by decolorization be referred to as a transparent toner hereinafter.
The decolorizable toner becomes transparent when subjected to light or heat. Hence, when a sheet carrying a toner image formed by a decolorizable toner sensitive to light is left in a light place, the toner image sequentially fades and, finally, becomes transparent. Also, a decolorizable toner image sensitive to heat becomes transparent when stored in a hot place. For this reason, the decolorizable toner is not feasible for images containing important data. It follows that if an image forming apparatus is operable only with a decolorizable toner, images to be stored have to be formed by another and exclusive image forming apparatus.
Moreover, although decolorization makes a decolorizable toner forming a toner image on a sheet transparent, it does not remove the toner image from the sheet; that is, the transparent toner image remains on the sheet even after decolorization. As a result, the amount of transparent toner the remaining on the sheet sequentially increases as the sheet is repetitively used, resulting in various problems, as follows. When a toner image is to be transferred to the sheet by a new image forming step, the transparent toner remaining on the sheet causes the image to be irregularly transferred, locally lost, or otherwise damaged. Further, the sheet with the transparent toner remaining thereon is apt to jam the transport path. In addition, in the event of fixation, hot offset or similar defects may occur. Therefore, despite decolorization, the number of times that a single sheet can be repetitively used is limited. Generally, the maximum number of repetitive use of a single sheet is considered to be 10 times at most.